1. Field of the Invention
This invention generally relates to methods and apparatus for connecting to an integrated circuit die, in particular where the die includes both analogue/microwave radio frequency (rf) circuitry and digital circuitry.
2. Background Art
One of the commonest methods of making connections from an integrated circuit die to an underlying package substrate bearing external interconnects for the integrated circuit is by means of wirebonding. With this technique a fine bond wire, typically of gold or aluminium, is used to connect a pad on the die to a pad on the package (or interconnect) substrate.
FIGS. 1a and 1b show a vertical cross-section and a view from above of a typical wirebonded integrated circuit 10 in which the die 12 is mounted on a ball grid array (BGA) substrate 14. Die pads 12a are connected to substrate pads 14a by means of a plurality of wirebonds 16 (omitted for clarity in the view from above), the substrate pads 14a being connected to solderballs 14b, which are arranged in a matrix, by meandering tracks (not shown). Typically the package substrate has four or more (conducting) layers to provide ground planes for impedance control as well as a sufficient number of layers for routing the tracks. Substrates may be fabricated from a variety of materials but typically organic (plastic) materials are employed, for example glass fibre with BT (bismalemide triazine) or FR-4/5 resin or polyimide. The die 12 is generally attached to substrate 14 by means of adhesive or underfill 18, typically epoxy. After wirebonding the die is encapsulated, typically using a glob top process.
Wirebonding techniques, although relatively popular and inexpensive, suffer from problems at higher frequencies, in particular microwave frequencies above 1 GHz because of the bond wire inductance. This can result in the bond wire having an undesirably high impedance at these frequencies. In particular the inductance of the bond wire forms a π-filter (C-L-C filter) with the capitance of the die bond pad 12a and the capitance of the package substrate, in particular the substrate pad 14a, as shown in FIG. 1c. A typical bond wire has a length of around 800 μm and conventional bondwire has an inductance of around 1 nH per millimetre of length so that the inductance of a typical bondwire is around 800 pH (an impedance, from the wire alone, of around 50 ohms). The π filter of FIG. 1c produces a low pass response with a typical cut-off frequency in the range 1 GHz to 10 GHz, which is a problem for many single chip mixed signal (digital and rf) devices, particularly as the rf operating frequencies increase above 1 GHz.
It is desirable to be able to reduce the length of a bond wire 16 to reduce the inductance and hence push the low pass filter cut off frequency above the relevant band of interest, but conventional methods of connecting to a die make this difficult. Referring again to FIG. 1, adhesive 18 forms a meniscus 20 which restricts how close substrate pads 14a can be placed to the die 12, hence restricting the minimum length of a bond wire 16. Moreover because the pitch of the die bond pads 12a is generally smaller than the minimum pitch of the substrate pads 14a, for example 70 μm as opposed to 100 μm, the bond wires are frequently fanned out beyond the perimeter of the die 12, as illustratively shown by dashed line 22 in FIG. 1b. 
It is known to mount a microwave integrated circuit in a cavity on a ceramic substrate to reduce the length of the bond wires, but this is expensive. Cavity-down BGA's where the chip (die) is mounted in a cavity in the underside of substrate 14, on a copper heat sink, are also known. The skilled person will be aware that a number of other chip packaging techniques are also available including flip-chip techniques (which have no bonding wires) and the so-called Sea of Leads (SoL) package (a variant on flip-chip with short, compliant chip interconnections). Another approach is to separate the rf and digital circuitry into two separate chips, but this has the disadvantage of requiring many connections between the two chips, which is expensive.